Electrodeposition of rhodium



ttesi 2,866,740 ELECTRODEPOSITION F RHODIUM Frank Herbert Reid,Hounslow, England, assignor to The International Nickel Company, Inc.,New York, N. Y., a corporation of Delaware No Drawing. Application May2, 1957 Serial No. 656,499

Claims priority, application Great Britain May 10, 1956 9 Claims. (Cl.204-47) thickness of a decora- 0.000005 and 0.00005 tween 0.0001 and0.002 inch or greater.

rhodium is deposited at the cathode.

Rhodium deposits as produced from the prior art baths show a pronouncedinternal tensile stress, which tions, but is generally of a high order,e. g., 50 to 100 In deposits thicker than about 0.0001 inch, the stressoften leads to cracking of the rhodium deposit in the as platedcondition. Cracks formed in this way may be referred to as primarycracks and usually extend right through the deposit to the basis metal.They may be detected by the so-called electrographic method, in whichthe basis metal, which is exposed by the cracking, pattern on a suitabletest paper. Cracks are of course undesirable, particularly when theeliminate this cracking tendency.

Although many attempts were made to overcome the foregoing diflicultiesand other disadvantages, none, as far as I am aware, was entirelysuccessful when carried into practice commercially on an industrialscale. It has now been discovered that by the use of novel rhodiumelectroplating baths, substantially crack-free electrodeposits may beproduced under normal commercial electroplating conditions. It is anobject of the present invention to provide a novel electrochemicalplating reduce or substantially eliminate cracks in rhodiumelectrodeposits. i Other objects and advantages will become apparentfrom the following description.

Generally speaking, the present inventioncontemplates the production ofelectrodeposits of rhodiunr through the use of an aqueous, acidicelectrodeposition' bath which contains a soluble rhodium salt and atleast sufficient free mineral acid to prevent hydrolysis of the selectedrhodium salt and selenium inan amount equiva:

to corrosion and to alia, as the surfaces inch, and that of anindustrial deposit is generally be The electro-flytes commonly used aresolutions of rhodium sulphate 30 bath which enables one to memo2,866,740 I Patented Dec. 30,1958

to provide an amount of selenate ion therein which is elfective inreducing the primary cracking tendency of the deposit. The solublerhodium salt suflicient topro vide from about 5- to grams per liter ofdissolved rhodium, for example, about 10 g. p. 1., may be selected fromthe group consisting of rhodium sulphate, rhodium] phosphate and rhodiumfluoborate. The free acidmay be selected from the group consisting ofsulphuric acid,"; phosphoric acid and fluoboric acid and maybe employedin amounts at least up to about 100 milliliterslpe liter (m. p. 1.). Thenecessary concentration of selenic 1 acid is supplied either by theaddition of the acid itself or by the addition of a salt. which uponhydrolysis in the presence of a mineral acid will produce a satisr'awtory effective quantity of selenic acid. 7 Of course, when adding thehydrolyzable salt, it is preferred thatthe reaction products of thehydrolysis reaction will not interfere with the electrodeposition.Accordingly, it is preferred to use a sodium or other alkali metalsaltof selenic acid when a salt addition is indicated.

In carrying the invention into practice, it is preferred to provide arange between a minimum of at least 0.05

g. p. l. to a maximum of about 1 g. p. l. (or even a maximum of about 8-g. p. l.) of selenic acid in the aqueous acidic rhodium electroplatingbath I prefer to add free selenic acid. Although any addition ofselenicacid will reduce the cracking to some extent, I prefer alwaystoadd at least 0.05 g. p. l. The amount necessary to ensure practicalfreedom from cracking under all conditions of deposition is about 0.4 g.p. l. in an electrolyte low in free mineral acid, e. g., sulphuric acid,and rather less in an electrolyte with a higher concentration of freeamount of free sulphuric acid. Further sulphuric acid.

is often added to increase the content of free sulphuric acid.

While it is to be observed ontemplates the minimization of primarycracking tendtheirsame criterion,-.a' current density of aboutl wherehardness of the deposit is an important charac teristic of the deposit.As was indicated hereinbefore,

to achieve optimum minimization of primary cracking tendency, the amountof selenic acid added to the electrolyte should be increased as theamount of free mineral acid is decreased. As a general rule, it ispreferred to adda minimum of 0.4g. p. l. of selenic acidto theelectrolyte regardless of the free mineral acid concentration.

it is to be observed that the present invention contemplates a moreadvantageous rhodium electrodeposition bath which contains ingredientsin the following tabulated ranges of concentration in units of g. p. l.

TABLE I Ingredients Broad Desired;

Rhodium (as soluble salt)..- about 10. Free Mineral Acid about 20.selenic Acid-.... about 1. Water Balance.

Mlllillters per liter Likewise, the, operating conditions are tabulated,in Table II:

TABLE II Conditions Broad Desired Temperature, C 2070.... 50 Current Denity, ampJdm. 05-2.... 1

It is to be observed that while the specification has disclosed that theelectrodeposits produced from the aforementioned baths of the presentinvention are of metallic rhodium, they contain smallamounts ofselenium, the,

amount thereof depending to a certain extent on the concentration ofselenic acid used in the bath. For purposes. of this specification, itis intended that the terms rhodium" or electrodeposited rhodium" whenused in reference to the electrodeposits include these small amounts ofselenium. In view of the inclusion or codeposition of selenium withrhodium, it should be noted that when the baths disclosed herein arereplenished, not only rhodium must be added thereto but also suitableadditions of selenic, acid should be made.

For the purpose of giving those skilled in the art a,

better understanding of the invention, the following illustrattveexamples are given:

Example I A rho'dium plating electrolyte was made by dissolving rhodiumhydroxide in sulphuric acid and diluting the Example II Electrolytecomposition: Rhodium '(as sulphate) gms./1iter Added sulphuric acidmls./liter 70 A deposit 0.0007 inch thick obtained from this electrolyteat temperature of 45 C. withaicurrentsdensity of 2 amp./dm. showedmoderate cracking. After addition of selenic acid to give aconcentration of 0.4 gm./liter, an uncracked deposit 0.0012 inch thickwas obtained.

Example Ill Example IV Flat copper specimens were plated on one sideonly with rhodium deposits 0.0005 inch thick from the electrolytes ofExample 111. The copper was then dissolved away by treatment with nitricacid. The deposit from the electrolyte that was free from selenic aciddisintegrated into numerous fine flakes because of cracks in it, but thedeposit from the treated electrolyte remained as a coherent foil.

The present invention is particularly applicable to theelectrodeposition of rhodium on any metal which is compatible with theacid electrolytes. These metals include nickel, nickel-silver, silver,gold, platinum, palladium, copper and brass.

It is to be observed that the present invention provides a process forthe production of deposits of rhodium having reduced cracking tendenciesfrom an acid electrolyte and also provides a novel acidic electrolytefor the production of such rhodium electrodeposits.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

I claim:

1. An electrolyte for producing substantially crackfree rhodIumelectrodeposits comprising an aqueous acidic bath containing dissolvedrhodium derived from at least one compound from the group consisting ofrhodium sulphate, rhodium phosphate and rhodium fiuoborate in an amountto provide from about 5 to about 20 grams per liter of rhodium, freeacid in an amount sulficient to prevent hydrolysis and a compound fromthe group consisting of selenic acid and alkali metal salts thereof inan amount equivalent to about 0.05 to about 25 grams per liter ofselenic acid.

2. An electrolyte for the production of substantially crack-free rhodiumelectrodeposits comprising an aqueous acid bath containing dissolvedrhodium derived from at least one compound from the group consisting ofrhodium sulphate, rhodium phosphate and rhodium fiuoborate in an amountto provide from about 5 to about 20 grams per liter of dissolvedrhodium, an acid from the group consisting of sulphuric acid, phosphoricacid and fiuoboric acid in an amount sufficient to prevent hydrolysis ofthe rhodium compound but not substantially exceeding about millilitersper liter of electrolyte and a compound from the group consisting ofselenic acid and alkali metal. salts thereof in an amount equivalent toabout 0.05 to about 8 grams per liter of selenic acid.

3. A rhodium plating electrolyte for producing hard, substantiallycrack-free rhodium electrodeposits comprising an aqueous acidic bathcontaining dissolved rhodium derived from at least one compound from thegroup consisting of rhodium sulphate, rhodium phosphate and rhodiumfiuoborate in an amount to provide about 10 gramsper liter of dissolvedrhodium, from about 12 to about 100 milliliters per liter of sulphuricacid and about 0.05 to about 1 gram per liter of selenic acid.

4. The process for producing substantially crack-free electrodeposits ofrhodium which comprises establishing an aqueous acidic electrolytecontaining about 5 to about 20 grams per liter of rhodium, from about 12to about 100 milliliters per liter of free acid selected from the groupconsisting of sulphuric acid, phosphoric acid and fluoboric acid, atleast about 005 up to about 1 gram per liter of selenic acid, andelectrolyzing said bath at a current density of about 0.5 to about 2amperes per square decimeter and a temperature of about 50 C. to producea rhodium electrodeposit characterized by being substantially free fromcracking.

5. The process for producing substantially crack-free electrodeposits ofrhodium which comprises establishing an aqueous acidic electrolytecontaining about 5 to about 20 grams per liter of rhodium, from about 12to about 100 milliliters per liter of free sulphuric acid and about 0.4gram per liter of selenic acid, and electrolyzing said bath at a currentdensity of about 1 ampere per square decimeter and a temperature ofabout 50 C. to produce a rhodium electrodeposit characterized by beingsubstantially free from cracking.

6. A rhodium plating bath for producing substantially crack-free rhodiumelectrodeposits comprising an aqueous acidic electrolyte containing insolution about 5 to about 20 grams per liter of rhodium, free acid in anamount sufficient to prevent hydrolysis of said rhodium and selenate ionin an amount equivalent to a concentration of at least about 0.05 up toabout 25 grams per liter of selenic acid.

7. A rhodium plating bath for producing substantially crack-free rhodiumelectrodeposits comprising an aqueous acidic electrolyte containing insolution rhodium derived from at least one compound from the groupconsisting of rhodium sulphate, rhodium phosphate and rhodium fluoboratein an amount to provide from about 5 to about 20 grams per liter ofdissolved rhodium, from about 12 to about 100 milliliters per liter offree acid from the group consisting of sulphuric acid, phosphoric acidand fluoboric acid and selenate ion in an amount equivalent to about0.05 to about 1 gram per liter of selenic acid.

8. The process for producing substantially crack-free electrodeposits ofrhodium which comprises establishing an aqueous acidic electrolytecontaining in solution about 5 to about 20 grams per liter of rhodium,free acid in an amount sufiicient to prevent hydrolysis of said rhodiumand selenate ion in an amount equivalent to a concentration of at leastabout 0.05 up to about 25 grams per liter of selenic acid andelectrolyzing said bath at a current density of about 0.5 to about 2amperes per square decimeter and a temperature of about 20 C. to aboutC. to produce a rhodium electrodeposit characterized by beingsubstantially free from cracking.

9. The process for producing substantially crack-free electrodeposits ofrhodium which comprises establishing an aqueous acidic electrolytecontaining in solution about 5 to about 20 grams per liter of rhodium,free acid selected from the group consisting of sulphuric acid,phosphoric acid and fluoboric acid in an amount suflicient to preventhydrolysis of said rhodium but not substantially exceeding aboutmilliliters per liter of electrolyte and a compound from the groupconsisting of selenic acid and alkali metal salts thereof in an amountequivalent to a concentration of at least about 0.05 up to about 8 gramsper liter of selenic acid and electrolyzing said bath at a currentdensity of about 0.5 to about 2 amperes per square decimeter and atemperature of about 20 C. to about 70 C. to produce a rhodiumelectrodeposit characterized by being substantially free from cracking.

References Cited in the file of this patent UNITED STATES PATENTS2,384,501 Streicher Sept. 11, 1945 2,461,933 Smith et al. Feb. 15, 19492,577,365 Reid Dec. 4, 1951 FOREIGN PATENTS 440,938 Great Britain Jan.8, 1936 OTHER REFERENCES Weisner et al.: Plating, vol. 43 (March 1956),pp.

1. AN ELECTROLYTE FOR PRODUCING SUBSTANTIALLY CRACKFREE RHODIUMELECTRODEPOSITS COMPRISING AN AQUEOUS ACIDIC BATH CONTAINIGN DISSOLVEDRHODIUM DERIVED FROM AT LEAST ONE COMPOUND FROM THE GROUP CONSISTING OFRHODIUM SULPHATE, RHODIUM PHOSPHATE AND RHODIUM FLUOBORATE IN AN AMOUNTTO PROVIDE FROM ABOUT 5 TO ABOUT 20 GRAMS PER LITER OF RHODIUM, FREEACID IN AN AMOUNT SUFFICIENT TO PREVENT HYDROLYSIS AND A COMPOUND FROMTHE GROUP CONSISTING OF SELENIC ACID AND ALKALI METAL SALTS THEREOF INAN AMOUNT EQUIVALIENT TO ABOUT 0.05 TO ABOUT 25 GRAMS PER LITER OFSELENIC ACID.